1. Field of the Invention
The present invention is related to a heat exchanger of a heater in which an electric heater is disposed integrally therewith to heat air in addition to hot water heated by a vehicle engine.
2. Description of the Related Art
Conventional heat exchangers having an integrated electric heater therein are disclosed in JP-A-5-69732 and JP-A-63-203411. A heat exchanger of a heater in which hot water or engine coolant is used to heat air is provided with an integrated electric heater. When the coolant temperature is low, for example when the engine is just started, the electric heater is turned on to generate heat, thereby heating air. This structure reduces pressure loss in the heating air blow system of the heater as compared with a structure having a separate PTC heater. Because the PTC heater has a positive temperature characteristic sharply changing the resistance thereof at a set temperature, it is not necessary to provide a temperature control circuit so that the driving circuit thereof can be made simple.
The electric heater is composed of a PTC element and electrodes and is soldered to a heat exchanger core. Therefore, the PTC element is exposed to high-temperature air for soldering (e.g. 600.degree. C. for soldering aluminum members) and, accordingly, the electric characteristic of the heater element may be damaged substantially.
In a common air conditioning system for a vehicle, a heat exchanger of a heater is disposed at a downstream side of a heat exchanger for cooling air to control reheating by the heat exchanger of the heater, thereby controlling temperature of the air blown into the passenger compartment of the vehicle. Therefore, condensed water formed on the heat exchanger for cooling air or snow coming from the air inlet may adhere the front surface of the heat exchanger of the heater. Because the electric heater is exposed to the outside from the heat exchanger core, the water or snow may cause short circuiting or electric leakage.
In the above conventional device disclosed in the publication, it is only disclosed that the set temperature of the PTC heater is 80.degree. C. There is no explanation about how to decide the set temperature. Our experiments have revealed that the heat generated by the PTC heater may not be utilized for the heating air to be heated if the set temperature of the PTC heater is not suitable.
In a core unit of a heat exchanger, a plurality of flat tubes for conducting water or engine coolant are parallelly disposed, and each of a plurality of corrugated fins is disposed between two of the flat tubes. If a PTC heater is installed in place of one of the flat tubes, the heat of the PTC heater is conducted via the corrugated fins and the adjacent flat tubes to the water. If the PTC heater is powered when the water temperature is low, temperature of portions of the corrugated fins adjacent to the PTC heater becomes higher than the temperature of portions of the corrugated fins adjacent to the flat tubes. If the set temperature of the PTC is too high, the heat generated by the PTC heater is transmitted to the water. That is, the PTC heater can not heat the heating air to be used for the heater effectively. On the other hand, if the set temperature is too low, the PTC heater can not generate power sufficient to heat the heating air.